Comparison of Flow Mechanism of Blade Sweep between a Transonic Single Rotor and a Rotor in Stage Environment

In order to reveal the different effect mechanisms of blade sweep on the aerodynamic performance when a transonic rotor operates alone or in fan stage environment, two series of forward and backward swept rotors were designed and utilized in the first stage of the dual-stage NASA CR-120859 fan. Results show that, the influence of sweep on the single rotor and the whole stage is different, indicating swept designs for rotor alone may not be suitable for the stage operations. The distinct effect of sweep is account for the difference of the flow field characteristic and stall mechanism of the single rotor and the rotor in the stage environment. The single rotor is tip limited and its stall mechanism is shock/tip leakage vortex (TLV) interaction, whereas the fan stage is hub limited and its stall mechanism is the severe corner separation at stage hub region. For the single rotor, forward sweep increases the stall margin (SM) for all sweep schemes, while backward sweep reduces it in general. For the fan stage, however, backward swept rotor significantly increases stall margin and the stall mechanism is changed to shock/TLV interaction. On the contrary, forward sweep reduces stall margin in general. The flow mechanism is that forward sweep reduces blade loading at tip region near leading edge (LE) and causes the shock to move downstream. Both the variations improve flow field at tip region, while backward sweep exerts an opposite effect. At hub region, backward sweep reduces radial flow tendency by varying radial pressure gradient, causing reduction of corner separation at rotor hub, while forward sweep enhances corner separation. Moreover, with increasing of swept height and swept angle, the chock mass flow, peak efficiency and total pressure ratio of forward sweep are reduced in general, while an opposite effect can be found for backward sweep.